Many catalytic processes exist for the polymerization or copolymerization of olefins such as ethylene and propylene. These processes have traditionally utilized a Ziegler-Natta catalyst system. These catalyst systems contain a transition metal compound (typically a titanium, zirconium, or vanadium halide or alkoxide) and a main group metal alkyl (usually an aluminum alkyl). The Ziegler-Natta catalyst systems are heterogeneous and possess a number of different active catalyst sites. Each different active site has different characteristics and produces a different polymer, and as a result, Ziegler-Natta catalyst systems produce polyolefins with broad molecular weight distributions and copolymers with broad compositional distributions.
Recent developments in the field of olefin polymerization have focused on the use of transition metal compounds having at least one .pi.-bound cyclopentadienyl ligand. The cyclopentadienyl ligand can be substituted or unsubstituted, and generally includes fused ring derivatives such as indenyl and fluorenyl. These cyclopentadienyl transition metal compounds are often referred to as metallocenes, though the term was initially used to describe biscyclopentadienyl compounds such as dicyclopentadienyliron (ferrocene).
Olefin polymerization systems using metallocenes differ from Ziegler-Natta catalyst systems in important ways. With metallocene catalysts, there is generally only one catalytically active species responsible for the polymerization of the monomers. The metallocenes, therefore, produce uniform chains of polymer having narrower molecular weight distributions and narrower compositional distribution. Metallocene catalysts are also typically much more active on a weight basis than Ziegler-Natta catalysts. Metallocene catalysts can be 10 to 1,000 times more active than the best Ziegler-Natta catalysts.
Metallocene catalysts are often classified into two separate groups, those possessing one cyclopentadienyl ligand, and those possessing two cyclopentadienyl ligands. The monocyclopentadienyl metallocenes are generally known in the art as good styrene polymerization catalysts and poor olefin polymerization catalysts, whereas biscyclopentadienyl metallocenes are generally known in the art as good olefin polymerization catalysts and poor styrene polymerization catalysts. Representative examples of these various catalysts are disclosed in the PCT patent application WO 96/13529; U.S. Pat. Nos. 4,978,730; 5,023,222; 5,045,517; 5,066,741; 5,196,490; 5,340,892; 5,554,795; 5,563,284; 5,565,396; 5,578,741; 5,591,874, and German AS 19602543.5, disclosing monocyclopentadienyl metallocenes. Examples of biscyclopentadienyl metallocenes are disclosed in U.S. Pat. Nos. 4,404,344; 4,542,199; 4,752,597; 5,198,401; 5,278,119; and 5,453,475.
However, many of the known metallocene catalysts are unstable under a variety of conditions, particularly when those conditions include exposure to water- or oxygen-containing media. This exposure can occur as a result of minor amounts of contaminants already present in the system, or later inadvertent exposure. This results in the decomposition and/or deactivation of the metallocene catalyst producing less than optimum productivity as well as erratic productivity. Due to this decomposition and/or deactivation, extreme handling conditions are required. This special handling adds to the final cost of these very expensive catalysts, making them less desirable in commercial polymerization processes.
In light of the above, it would be very desirable to have metallocene catalysts that are stable under a wide variety of conditions, particularly under conditions that include exposure to water or oxygen containing media.